1. Field of the Invention
The present invention relates to line scan photo sensors. In particular, the invention relates to a sensor architecture and method that increases the photo response by time delay and integration without compromising the sensor's blue and UV response.
2. Description of Related Art
In a line scan sensor, an image conjugate is scanned across a linear array of sensor pixels and/or a linear array of sensor pixels are scanned across an image conjugate. To achieve a higher speed sensor under the same ambient light conditions, the sensor must have pixels with a higher response to the same ambient light. Known sensors increase the response to ambient light conditions using a TDI architecture (Time Delay and Integrate architecture). TDI sensors are typically formed in a rectangular format where the image conjugate to be sensed moves across the face of the sensor pixels at the exact same rate that the photo charge accumulating in the pixels is transferred along the direction of a vertical CCD register so that the end of the vertical CCD register includes the accumulated photo charge generated by a single image area but accumulated over an extended time.
The photo charge in a TDI CCD sensor is clocked along the CCD registers under the influence of electric fields caused by clock voltages applied to the CCD gate electrodes, typically formed of doped polycrystalline silicon (commonly referred to as poly). The doped polycrystalline silicon conducts electrical signals, and infrared light passes through the doped polycrystalline silicon with ease; however, blue light and/or ultraviolet light are absorbed in the polycrystalline silicon layers. Such typical TDI CCD sensors lack a good response to blue light and/or UV light.
To improve response to blue light and/or UV light, linear arrays of photodiodes, and/or pinned photodiodes (PPD) are commonly used. Photodiodes and PPD's have good response to blue light and/or UV light at least in part because they are not covered by poly layers. Therefore, known TDI architectures are not useable.
In known sensors, the approach to improved response has been to try to increase the charge conversion efficiency (CCE) in a single linear array of photodiodes and/or PPD's. The CCE is process dependent, and a very good (and expensive) process is required to achieve very high CCE. Increasing the CCE also reduces the full well capacity, resulting in higher photon shot noise at saturation. The architecture of the present invention provides a two times increase in the response to light and a 30% reduction in photon shot noise in the sensor.